I'm really seeing good potential in your game.
Here's some advice:
-I see that there is a upgrade system in your game. Honestly it is fine without them. The game is beatable without them(though it's quite hard & you need to know how to stall the early game). It would take too much time to program the skills plus testing, so it would increase the development time to quite an extent.
As for how to balance the game:
-increase the columns for like either 1 - 2 tiles. This would make the monster girls reach the end much longer, more space for units, plus the early game stall technique will be much easier to execute
-increase projectile speed more specifically even just the wizard. It still takes ages for the projectile from the back to hit the front and the monster girl could like walk for 1 tile before being hit.
Some comments:
-the change to the witch is much appreciated since like she's way too op before. The game noticeably becomes easier a bit cause the shield guy now couldn't get oneshotted unlike before.